This is to make room in the api for quadratic curves.
In detail:
- Replace GSK_PATH_CURVE by _CUBIC
- Rename gsk_path_builder_curve_to to _cubic_to
- Replace GSK_PATH_FOREACH_ALLOW_CURVE with _CUBIC
- Replace GskCurveCurve with GskCubicCurve
When decomposing curves that are too straight, we may emit lines for
long parts of the curve. These lines do not properly map
t => distance
and it is better to treat them as a regular line than a curve.
This reason argument gives that information.
No users so far, that will happen in followup commits.
